1. Field of the Invention
The present invention relates generally to a method of providing a video call service in a mobile station, and in particular, to a method of providing a video call service in a mobile station so as to preferentially guarantee an audio quality of service (QoS) in a weak signal environment during a video call.
2. Description of the Related Art
Commonly, a video call service in a transmitting mobile station is achieved by compressing audio and video (A/V) data and transmitting it to a receiving mobile station. The A/V data is compressed to transmit more data in a given bandwidth. In such a video call method using a mobile station, control data is inserted in the compressed A/V data using a data service function and transmitted/received through a traffic channel. However, the reliability of the QoS is not guaranteed due to a delay and jitter resulting from the channel characteristics. Thus, a predetermined protocol is used in a transmitting mobile station to realize real-time streaming by detecting a data loss occurring in such a transmitting/receiving process.
A/V data is mainly damaged during transmission in a wireless environment, and to correct for the damage, the A/V data is transmitted with control data to identify an error using a protocol such as Real-time Transport Protocol (RTP), H.324M, H.323, or Session Initiation Protocol (SIP). In particular, H.323 that is an International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) video call standard is a protocol to transmit compressed A/V data in a Wideband Code Division Multiple Access (WCDMA) based transmitting mobile station. According to some of the above-described protocols, data loss can be recovered through re-transmission. However, since a video call needs real-time streaming, the protocols using the re-transmission technique are not used, and data is only minimally corrected using error correction code or discarded in a case where an error is discovered as a result of an integrity check.
In the WCDMA, H.324M is used to perform a video call, so the description will proceed based on H.324M, a 3rd generation (3G) video call standard protocol. A process of providing a video call service will now be described in detail with reference to FIGS. 1 and 2. FIG. 1 is a diagram for explaining a method of providing a video call service in a conventional transmitting mobile station, and FIG. 2 is a diagram for explaining a method of performing a video call in a conventional receiving mobile station.
Referring to FIG. 1, according to H.324M, every MUX-PDU (Multiplex-Protocol Data Unit) 20 is created by multiplexing encoded audio data 10 with video data every 20 ms. Then, the transmitting mobile station transmits the MUX-PDUs 20 to an air network through traffic channels S10 to S60 at a preset time interval. In the receiving mobile station, audio data and video data are separated from A/V data transmitted through the channels S10 to S60 by demultiplexing the transmitted A/V data as illustrated in FIG. 2, and the separated audio data and video data are transmitted to a codec decoder outputting audio and video signals.
However, a video call in a handoff area or in a weak signal environment area in which the strength of an electronic wave is weak experiences occasional video freezing and audio interruption. For example, while a MUX-PDU can be normally received in a good channel state of the channel S10 and audio data A0 can be reproduced for 20 ms, audio reproduction cannot be achieved for 40 ms due to loss of MUX-PDUs in bad channel states of the channels S20 and S30, and then when a normal MUX-PDU is received through the channel S40, audio reproduction can be achieved for 20 ms. Likewise, according channel states, audio interruption occurs by obtaining audio data A0, A3, and A5 among entire audio data A0 to A5 as illustrated in reference numeral 30 of FIG. 2.
From the point of view of a user, since an image output using previous data is continuously displayed even if video data is damaged, the user can continue to view the other party's face although it is a frozen image. However, audio interruption considerably degrades the service, and if the audio interruption exceeds a predetermined level, the audio is hardly audible. This phenomenon makes QoS of video data seem better than QoS of audio data from the point of view of the user. However, from the point of view of a mobile station transmitting/receiving data via the air, since audio and video data is combined single data, the QoS of the audio and video data are the same. That is, damage to A/V data means that both the audio data and the video data are damaged, and since there is no method of performing quality management by separating the A/V data, reliability of QoS of both the audio data and the video data is degraded.